When preparing OE devices like organic field-effect transistors (OFETs) or OPV cells, in particular flexible devices, usually printing techniques like inkjet printing, roll to roll printing, slot dye coating or gravure printing are used to apply the OSC layer. Contact printing techniques like gravure printing operate at high speed. However, high speed coating of a plastic substrate with an OSC ink or fluid can lead to a build up of static charge if the fluid is not conducting. This can lead to an electrostatic discharge by arcing, and, if the solvent is flammable, result in a fire or explosion. This hazard can be reduced by engineering solutions such as the use of tinsel and electrostatic neutralization bars. However, the rapid pumping of a non-conductive flammable fluid to a coating or printing head can also lead to electrostatic discharge.
Another possibility to reduce or avoid the building of static charge is to use conducting solvents. The static charge is then harmlessly dissipated to earth via contact with conductive surfaces on the printer. As a result, no static charge accumulates and arcing does not occur. However, this can put serious restraints on the possible choice of solvents for the OSC fluid. For example, the limited solubility of polythiophene-fullerene composites for the OSC fluid for printed bulk heterojunction polymer solar cells requires the use of solvents such as o-xylene [see Waldauf et al., Appl. Phys. Lett. 89, 233517 (2006)]. However, this solvent is virtually non-conductive and will therefore imply the above-mentioned problems due to static charge.
The inventors of the present invention have found that, in order to ensure the safety of printing and coating operations, it is possible to include a conductivity enhancing additive to the semiconductor fluid. The conductivity of the resultant fluid should be on the order from 2×10−6 to 1×10−8 Siemens/meter (S/m). The concentration of the additive should be as low as possible. The additive should not adversely affect the performance and lifetime of the devices.
Adding a conductive additive to an OSC material is described in prior art as a measure to increase conductivity of the semiconductor. However, when using a fluid comprising standard OSC materials, like poly(3-hexylthiophene), in an aromatic hydrocarbon solvent, it was so far not possible to achieve the required conductivity without permanently doping the polymer (for example with iodine or other oxidants). For the uses of the present invention, however, permanent doping is undesired as it would lead to a deterioration of the OSC device performance.
For example, US 2006/0175582 discloses a composition for preparing hole injection layers (HIL) or hole transport layers (HTL) for electroluminescent devices. The composition comprises for example a conjugated polymer, like e.g. poly(3-substituted thiophene), a solvent and an oxidant. The oxidant is used to permanently dope the polymer in order to increase its conductivity. Accordingly US 2006/0175582 suggests to use oxidants preferably in high concentrations and selected from highly oxidising additives and/or additives that will remain in the polymer after processing. However, this is exactly the effect that should be avoided by the materials and methods used in the present invention.
EP 0 822 236 A1 discloses a composition comprising a film-forming polymer matrix, an intrinsically conductive polymer dispersed in said matrix, and a material that controls the conductivity in said composition, which is selected from the group consisting of amines, ammonia, organic hydroxyl compounds, epoxides, ethoxylated and propoxylated compounds, acrylates, methacrylates, surfactants with a pH greater that about 7 and mixtures thereof. These materials are used to increase the conductivity of the deposited film or coating of the conductive polymer, and can also be added to the polymer blend after film formation. Again, this is what should be avoided by the materials and methods used in the present invention
It is therefore desirable to have fluids comprising an OSC that are suitable for the preparation of OE devices, especially OPV cells, which allow a broader selection of possible solvents, do not lead to problems of static charge as mentioned above, and will not lead to permanent doping of the OSC or otherwise adversely affect the performance and lifetime of the device. One aim of the present invention is to provide such improved fluids. Another aim is to provide improved methods of preparing an OE device from such fluids. Another aim is to provide improved OE devices obtained from such fluids and methods. Further aims are immediately evident to the person skilled in the art from the following description.
The inventors of the present invention have found these aims can be achieved, and the above-mentioned problems can be solved, by providing methods, materials and devices as claimed in the present invention, especially by providing a process for preparing an OE device using a low conducting ink based on a non-conducting solvent. In particular, the inventors of the present invention have found that it is possible to provide an ink with a low conductivity, which is sufficiently high to avoid the building of static charge in the printing process used for depositing the OSC onto the OE device, but is also sufficiently low to avoid a significant negative influence on the OE device performance. This is achieved by providing an ink comprising an OSC material and a non-conducting organic solvent, preferably an aromatic solvent, and further comprising a small amount of one or more conductivity enhancing agents, i.e. additives that increase the conductivity of the formulation (hereinafter also shortly referred to as “conductive additives”). The conductive additive used is either volatile, so that it is evaporated together with the solvent after deposition of the OSC layer on the device, and is thus not remaining in the OSC layer. Alternatively the conductive additive used does not have an oxidising effect on the OSC material. As a result, permanent electrical doping of the OSC material, which could render the OSC too conductive and thereby adversely affect the desired OE device properties, is avoided.